A flush joint tubular connection has inner and outer diameters substantially the same as the tubing joints which the connection connects.
A flush joint tubular connection made by the Hydril Company and covered by numerous patents comprise a first straight thread, a second straight thread of sufficient diameter to pass within the bore of the first thread and a tapered mating seal between the two joints of tubing which is a premium joint of high cost and according to published data, enjoys only 42% axial strength with regard to the pipe wall.
Standard A.P.I. non-upset tubing connections comprise couplings having outer diameters considerably larger than the pipe outer diameter but still only enjoy approximately 42% efficiency as above. A.P.I. does list a "turned down" collar outer diameter to increase clearance between strings, however, the "turned down" diameter still exceeds substantially, the pipe outer diameter.
No prior art discloses a flush joint tubular connection having tapered threads, that when properly assembled, effects optimum stresses within the small end of the external thread and within the large end of the internal thread so as to provide a connection of maximum efficiency. Conventional pipe connections have threads with like tapers and result in a constant diametrical interferance along the taper between the external and internal threads, thereby causing excessive stresses or requiring increased wall thickness at the end of the pipe. Excessive stresses reduce the joint strength and an increased wall thickness rules out a flush joint connection.
It is therefore clear that a flush joint connection having a high efficiency as provided by the instant invention is nedded for use within oilwells and other pipe assemblies wherein radial clearance is limited.
Standard pipe threads as well as A.P.I. threaded connections have such a tendency to cross-thread that "stabbing guides" are often used at a considerable cost of time and expense. Such threads have an extremely shallow stab depth and a relatively large thread depth, both of which add to the cross-thread problem. Perfect alignment is difficult to attain under normal field conditions and often impossible to attain under difficult conditions. Premium connections such as disclosed by Stone in U.S. Pat. No. 1,932,427 require even closer alignment to stab because of the close fit of straight threads and the "pin nose" seal 32 which is highly suseptable to damage. To applicant's belief no prior art comprises the combination of a deep stab, thread height and thread diameter as required to provide a tapered threaded connection that will stab easily and quickly without the possibility of cross-threading. By way of an example, a 23/8 EU 8rd A.P.I. tubing thread has a 2.473" pin end diameter and a 2.437" box bore at the first thread which allows no entry of the pin into the box at stab position. The counterbore of the box allows entry of the pin only 0.446" affording at best, axial alignment but no angular alignment so less than six degrees of angular misalignment will allow it to cross-thread.
About 1940, A.P.I. changed from 10 V threads to 8rd and a substantial improvement resulted because less gauling occurred during makeup of the threads. It was then commonly assummed "that any thread finner than 8 threads per inch would gaul and cross-thread" and that myth persists today. However, the improvement resulted almost entirely from the better thread form, eliminating the sharp egded V threads. The present invention with threads as fine as 20 per inch, run fast and smooth without cross-threading, and it has other features as well.